The recent development and practical implementations of high data rate fiber optic communication systems for carrying multi-megabit/sec. digital data, such as high density multiplexed fiber optic telephone trunking systems, have included refinements in the field of optic signal transmission components, such as optical regenerator circuitry and electro-optic transmitter devices. A common requirement in these systems is the stability of the optical pulse transmitter components, typically injection laser diodes (ILDs) which are turned on and off at very high data rates to generate the required optical representations of the electrical signal inputs to be conveyed over a fiber optic highway to a repeater (regenerator) or receiver station.
Usually, data to be transmitted is applied to an ILD with a feedback control circuit sensing the optical output power and generating a control signal in an effort to maintain the average optical output power constant. An injection laser diode transmitter configuration illustrative of this type of device is described in the U.S. patent to Maione et al U.S. Pat. No. 4,019,048 and on pages 1824-1828 of the Bell System Technical Journal, July-August, 1978. Through a prebias feedback control arrangement, fluctuations of the laser optical output caused by temperature variations or aging result in a corresponding change in photo-current produced by a relatively slow optical output-monitoring photo diode. This photo-current represents an average of the peak laser diode output integrated over the time constant of the photo diode and therefore provides a slowly variable DC control level for compensating for any drifting of the optical output of the laser diode due to temperature and aging.
In copending application Ser. No. 149,291, filed May 12, 1980, by P. Casper et al, entitled MultiChannel, Repeatered, Fiber optic Communication Network and assigned to the assignee of the present application, there is described a system for monitoring the quality of signals transmitted over the fiber optic communication links and isolating the location of a fault of source of signal degradation along one of the fiber optic links. Advantageously, the system described in the above-referenced application employs an electro-optic transmitter according to the present invention which provides an extremely accurate and simple technique for monitoring the signal quality along a fiber optic link, while, at the same time, providing the stability required to offset the effects of temperature fluctuations and aging.